In these days, to enhance the work efficiency of welding, automatic welding using a welding robot has been employed. FIG. 10 shows the typical structure of a welding robot which is an articulated robot. As shown in the figure, a welding torch 3 is mounted to an end of a wrist portion 2 of a manipulator 1. A welding wire 5 is wound on a wire reel 4 and fed to the welding torch 3 by a wire feeder 6 mounted to the manipulator 1. Electric power is supplied from a welding power source 7 to the welding torch 3. Shielding gas is supplied from a gas cylinder 8 to the welding torch 3. Command signals are inputted from a teach pendant 9 into a robot controller 10. Signals from the robot controller 10 are inputted into the manipulator 1. Thus, the position of the end of the welding torch 3 is controlled by rotating six shafts made up of a first shaft through a sixth shaft.
The conventionally used welding torch 3 will be described below with reference to the sectional view of FIG. 11. As shown in the figure, a tip body 13 is mounted to a torch body 12. A power feed tip 14 is mounted to the front end 13a of the tip body. Each of the torch body 12, the tip body 13 and the power feed tip 14 is formed with a wire insertion hole at the axial center. The welding wire 5 is inserted to pass through the wire insertion holes to come into contact with the interior of the power feed tip 14, whereby electric power is supplied to the welding wire 5.
An orifice 17 is provided at a lower portion of the tip body 13. A nozzle 16 is provided to surround the power feed tip 14 and the orifice 17. The shielding gas is belched through an injection hole 17a provided at the orifice 17. The belched shielding gas shields the arc, the molten pool and the nearby portion from nitrogen and oxygen in the air. An insulating bush 18 is provided around the tip body 13.
Since the welding wire 5 to be fed to the welding torch 3 is generally wound on the wire reel 4, the welding wire 5 has a tendency to be bent. Since such a bending tendency of the welding wire 5 is not uniform, it is difficult to position the front end of the welding wire 5 being fed out of the power feed tip 14 accurately at the intended welding position. Further, even when the bending tendency of the welding wire 5 is made uniform before the welding wire is fed to the welding torch 3, there still is a problem. That is, when the posture of the robot is changed in a complicated way, the wire guide liner (not shown) for guiding the welding wire 5 is twisted, so that the welding wire 5 to be fed to the welding torch 3 is twisted. As a result, the front end of the welding wire 5 cannot be accurately positioned at the intended welding position. Thus, uniform and beautiful weld beads cannot be obtained.
As noted before, electric power is supplied to the welding wire 5 by bringing the welding wire 5 into contact with the interior of the power feed tip 14. Herein, when the bend of the welding wire 5 is not uniform, the contact point between the welding wire 5 and the power feed tip 14 is not fixed. As a result, the length of the projecting portion of the welding wire (i.e., the distance from the contact point between the welding wire 5 and the power feed tip 14 to the front end 5a of the welding wire 5) varies. Thus, the resistive heating of the projecting portion of the welding wire 5 varies, so that the melting amount of the welding wire 5 varies. As a result, the arc length varies, so that welding cannot be performed uniformly.
When the welding wire 5 is stored in a pail pack instead of the wire reel 4, the degree of the bending tendency of the welding wire 5 is relatively small. Thus, the contact point between the welding wire 5 and the power feed tip 14 varies, which may cause a spark to be produced between the welding wire 5 and the inner surface of the power feed tip 14. Due to the heat of the spark, the power feed tip 14 wears out or the welding wire 5 is fused to the inner surface of the power feed tip 14. As a result, the lifetime of the power feed tip 14 is shortened, and the power feed tip 14 needs to be replaced frequently. It has been demanded that the number of times the operation of the robot is stopped for replacement of the power feed tip 14 and so on is reduced, and the conventional structure does not satisfy the demand.
To solve the above-described problems, another welding torch has been proposed. In this welding torch, the power feed tip includes an outer tip and an inner tip. The inner tip is formed with a vertical slit extending from the front end along the wire insertion hole. The inner tip is pressed by a spring provided in the tip body (see Patent Document 1, for example). As will be described below, however, this arrangement cannot reduce the influence of the arc heat on the inner tip, so that the lifetime of the inner tip cannot be increased.
Patent Document 1: JP-A-2002-59265
It is said that heat of thousands to tens of thousands of degrees is generated at the welding point. Due to the radiant heat, the front end of the power feed tip is heated to reach a high temperature of almost hundreds to a thousand of degrees. Since the power feed tip is generally made of a copper alloy, the power feed tip reduces the surface hardness and softens as the temperature rises. As a result, the abrasion of the power feed tip progresses at an accelerating pace. Thus, the lifetime of the power feed tip cannot be increased.